The effect of Tinospora crispa extracts as a natural mild steel corrosion inhibitor in 1 M HCl solution

2.1 Preparation of T. crispa extract

T. crispa stems were collected around from Gurun, Kedah, Malaysia. T. crispa were washed under the running tap water and then were cut into small pieces, which were about 1 cm each piece. T. crispa stems in small pieces were then sun dried for seven days to remove water before being ground into fine powder and sieved with a 250 μm mesh. Initially, the dried powder was diluted with distilled water in the ratio of 1:10 (w/v). The solution was then placed in a water bath at 80 °C for 24 h and then filtered to obtain the supernatant. The collected supernatant was then freeze dried for at least 3 days. The dried extract obtained was ground into powder and labeled as TCDW, was kept at 4 °C for further use. The same extraction process goes to the next extracting solvent. The dried powder was diluted with 70% acetone solution (v/v) in the ratio of 1:10 (w/v). The collected supernatant was then concentrated at 45 °C under reduced pressure in a rotatory evaporator and dried in an oven at 50 °C. The dried extract obtained was ground into powder and labeled as TCAW, was kept at 4 °C for further use.

2.2 Specimen preparation

Mild steel specimen having the composition (wt%) of 0.08 C, 0.01 Si, 1.26 Mn, 0.02 P and remaining Fe was used. Coupons were cut into 3 × 4 × 0.1 cm dimensions used for weight loss measurements, whereas specimens with 6 × 4 × 0.1 cm dimensions were used as working electrode for polarization and EIS measurements. The exposed area was mechanically abraded with 400, 600 and 800 grades of SiC papers, degreased with isopropyl alcohol (IPA) and rinsed with distilled water before each electrochemical experiment.

2.3 Solutions preparation

About 1 M HCl solutions were prepared by the dilution of 37% HCl using distilled water. The concentration range of T. crispa extract employed was varied from 200 to 1000 ppm. This concentration range was chosen upon the maximum solubility of T. crispa extract. The powder extract was first dissolved in 1% (v/v) methanol before it was diluted with 1 M HCl solution.

2.4 Weight loss measurement

The rectangular mild steel specimens of dimension 6 × 4 × 0.1 cm were immersed (complete immersion) in 100 mL of deaerated electrolyte in the absence and presence of different concentrations of T. crispa extracts at an ambient temperature (303 K). The weight loss of mild steel specimens was determined after 24 h of immersion. The percentage inhibition efficiency (IE%) was calculated using the following formula:

2.5 Potentiodynamic polarization measurement

Polarization measurements were conducted in a conventional three electrode Pyrex cell with an overall volume of 50 mL. The exposed geometrical surface area of the working electrode (WE) was fixed with an area of 0.785 cm² to the electrolyte, a graphite gauze was used as the auxiliary electrode (CE), and a saturated calomel electrode (SCE) as the reference electrode (RE). All the experiments were carried out using deaerated unstirred solutions at 303 K. The measurements were carried out using a PC controlled Volta Lab PGP 201 system with Voltamaster 4 software at a scan rate of 1 mV s^–1. The open circuit potential, E_ocp was measured for 30 min to allow the stabilization of the steady state potential. The potential range was calculated from the E_ocp values obtained (±250 mV). The inhibition efficiency (IE%) was calculated using the relation:

2.6 Electrochemical impedance spectroscopy (EIS)

The electrochemical impedance spectroscopy (EIS) was carried out using a Gamry Instrument Reference600 with the open circuit potential E_ocp, of every sample was immersed for 30 min over a frequency range of 100 kHz to 0.01 Hz with a signal amplitude perturbation of 5 mV and a scan rate of 1 mV s^–1. Next, it was fitted with sets of circuits using Echem Analyst software that gave the best value. The inhibition efficiency (IE%), was calculated by the following equation:

2.7 Surface analysis

The surface morphology of steels specimens was evaluated by a scanning electron microscope (SEM) analysis (Leo Supra 50VP). A test specimen that exhibits higher efficiency of corrosion inhibition from weight loss measurements was examined with scanning electron microscopy (SEM) instead of blank (without inhibitor) and fresh steel.

3.1 Weight loss measurement

Weight loss measurement is a non-electrochemical technique for the determination of corrosion rates and inhibitor efficiency which provides more reliable results than electrochemical techniques because the experimental conditions are approached in a more realistic manner yet the immersions tests are time-consuming (Hussin and Kassim, 2011; de Souza and Spinelli, 2009). Therefore, due to such differences (experimental conditions), the values would obviously differ from the electrochemical values. Table 1 shows the percentage of inhibition efficiency (IE) for T. crispa extracts at the temperature of 303 K. From the table, it was revealed that as the concentrations of the inhibitor increased, the percentage of inhibition efficiency was also increased (concentration-dependent). The optimum value of inhibition was obtained at the concentration of 800 ppm (for TCDW) and 1000 ppm (for TCAW) respectively. This indicates that the adsorption process between the adsorbate (inhibitor) and the mild steel surface was efficiently achieved and led to the formation of a strong metal–inhibitor interaction thus lowering the capability of chloride ion (Cl⁻) to adsorb on the mild steel surface. In turn, the situation changes in contrast for the TCDW extract if the concentrations exceed 800 ppm thus explaining the reduction in inhibition efficiency value. The surface coverage of the mild steel was also reduced upon the addition of T. crispa extracts which indicates that the extract acts as a good corrosion inhibitor for mild steel in acid solution (Fig. 1).

Close

Figure 1

The relationship between percentage of inhibition with concentration of T. crispa extracts.

Export to PPT

3.2 Potentiodynamic polarization measurement

Figs. 2 and 3 represent the potentiodynamic anodic and cathodic polarization plots (or known as Tafel plot) for mild steel in 1 M HCl with the absence and presence of both T. crispa extracts (TCDW and TCAW). In general, the Tafel plots gives useful information such as the corrosion potential (E_corr), corrosion current density (I_corr), resistant polarization (R_p) and the corrosion rate (CR) as summarized in Table 2. It was illustrated from the data that the addition of T. crispa extracts decreased the corrosion current density (I_corr). The decrease may be due to the adsorption of the inhibitor on metal/acid interface (Ahamad et al., 2010). The maximum inhibition was obtained at the concentration of 800 ppm (78.14% IE) and 1000 ppm (73.33% IE) for TCDW and TCAW, respectively. This result was in good agreement with the weight loss measurement which have been discussed above. Moreover, the decrease of I_corr of all T. crispa extracts in comparison with 1 M HCl for both the cathodic and anodic sites may suggest the mixed type corrosion inhibition behavior with a predominant decrease at the anodic site. The confirmation was also supported by the great difference of the anodic Tafel slope (β_a) compared with the cathodic Tafel slope (β_c) in the absence and presence of T. crispa extracts in 1 M HCl solution. Furthermore, the displacement of E_corr was <85 mV, hence the inhibitor can be seen as a mixed type inhibitor (Satapathy et al., 2009). This also indicates that the inhibitor merely blocks the reaction sites of the mild steel surface and changes the mechanism of metal dissolution (anodic) and/or hydrogen evolution (cathodic) reaction.

Close

Figure 2

Potentiodynamic polarization curves for mild steel in 1 M HCl containing different concentrations of TCDW.

Export to PPT

Close

Figure 3

Potentiodynamic polarization curves for mild steel in 1 M HCl containing different concentrations of TCAW.

Export to PPT

3.3 Electrochemical impedance spectroscopy (EIS)

The corrosion behavior of mild steel in 1 M HCl solution in the presence of T. crispa extracts was investigated by EIS at room temperature after 30 min of immersion. The Nyquist plots for mild steel obtained at the interface in the presence and absence of T. crispa extracts at different concentrations are given in Figs. 4 and 5. As observed, the Nyquist plots contain a depressed semi-circle with the center below the real X-axis, which is size increased by increasing the inhibitor concentrations, indicating that the corrosion is mainly a charge transfer process (Aljourani et al., 2009) and the formed inhibitive film was strengthened by the addition of T. crispa extracts. A loop is also seen at low frequencies (LF) which could arise from the adsorbed intermediate products such as (FeCl⁻)_ads in the absence of the inhibitor and/or (FeCl⁻Inh⁺)_ads in the presence of the inhibitor (Solmaz et al., 2008). The depressed semi-circle is the characteristic of solid electrodes and often refers to the frequency dispersion which arises due to the roughness and other inhomogeneities of the surface (Bentiss et al., 2005). It is worth noting that the change in concentration of T. crispa extracts did not alter the style of the impedance curves, suggesting a similar mechanism of the inhibition is involved. The impedance parameters derived from these plots are shown in Table 3. The constant phase element, CPE (Fig. 6) is introduced in the circuit instead of pure double layer capacitor to give a more accurate fit (Noor and Al-Moubaraki, 2008). The CPE, which is considered as a surface irregularity of the electrode, causes a greater depression in the Nyquist semi-circle where the metal/solution interface acts as a capacitor with irregular surface (Satapathy et al., 2009). The impedance of the CPE is expressed as

Close

Figure 4

Nyquist plots for mild steel in 1 M HCl containing different concentrations of TCDW.

Export to PPT

Close

Figure 5

Nyquist plots for mild steel in 1 M HCl containing different concentrations of TCAW.

Export to PPT

Close

Figure 6

Equivalent circuit for the impedance spectra.

Export to PPT

3.4 Adsorption isotherm and mechanism of inhibition

Adsorption isotherms provide information about the interaction of the adsorbed molecules with the electrode surface (Noor and Al-Moubaraki, 2008; Avci, 2008). The adsorption of organic adsorbate at the metal/solution interface can be presented as a substitution adsorption process between the organic molecules in aqueous solution (Org(sol)), and the water molecules on the metallic surface (H₂O(ads)) (Naderi et al., 2009):

Close

Figure 7

Langmuir adsorption plot for mild steel in 1 M HCl containing different concentration of (a) TCDW and (b) TCAW.

Export to PPT

The free energy of adsorption (ΔG_ads) can be calculated from the K_ads value obtained from the above correlation:

where 55.5 is the concentration of water, R is the universal gas constant and T is the absolute temperature. The adsorption–desorption equilibrium constant K_ads, was determined as 106.16 M⁻¹ and 55.81 M⁻¹, leading to ΔG_ads = −21.87 kJ mol⁻¹ and −20.25 kJ mol⁻¹ for TCDW and TCAW extract at the temperature of 303 K. The large K_ads value gives better inhibition efficiency due to strong electrical interaction between the double layer and adsorbing inhibitor molecules while a small K_ads value compromise that such interactions between the adsorbing inhibitor molecules and the metal surface are weaker, indicating that the inhibitor molecules are easily removable by the solvent molecules from the surface (Nasshorudin, 2010). The negative sign of ΔG_ads indicates the spontaneity of the adsorption process and stability of the adsorbed layer on the electrode surface (Hussin and Kassim, 2011; Aljourani et al., 2009). Generally, the values of ΔG_ads around −20 kJ mol⁻¹ or less negative are known to be associated with physical adsorption (electrostatic interactions between the inhibitor and charged surface) while those around −40 kJ mol⁻¹ or more negative is are known to be associated with chemisorption (charge sharing or transferring from organic molecules to the metal surface and form a coordinate type of metal bond) (Benali et al., 2007). From this estimation, it can be concluded that both T. crispa extracts (TCDW and TCAW) are physically adsorbed on the charged mid steel surface thus creating an electrostatic interaction.

In aqueous acidic solution, the organic molecules of T. crispa extract exist either as neutral molecules or in the form of protonated organic molecules (cation). These organic molecules may adsorb on the metal/acid solution interface by one or more of the following ways; (i) electrostatic interaction of protonated organic molecules with (FeCl⁻)_ads, (ii) donor–acceptor interactions between the π-electrons of aromatic ring and vacant d-orbital of surface iron atoms, (iii) interaction between unshared electron pairs of heteroatoms and vacant d-orbital of surface iron atoms (Ahamad et al., 2010).

3.5 Surface analysis

From the SEM photograph, the morphology of the corroded mild steel surface can be seen clearly. Based on Fig. 8a and b, the comparison between the untreated mild steel specimen and the mild steel specimen in 1 M HCl can be made. The mild steel specimen in 1 M HCl has a rough surface due to localized attack of hydrochloric acid thus forms a rust product on the surface. When a maximum concentration of the inhibitor is added to aqueous acid solution, the mild steel surface becomes significantly smooth (Fig. 8c and d) and comparable to the untreated mild steel surface (Fig. 8a).

Close

Figure 8

SEM pictograph of (a) untreated mild steel, (b) mild steel treated with 1 M HCl, (c) mild steel treated with 800 ppm TCDW and (d) mild steel treated with 1000 ppm TCAW at magnification of 100×.

Export to PPT